Field of the Invention
This invention relates to a rubber adherable steel cord adapted for reinforcement of resilient articles such as rubber hoses, rubber belts or vehicle tires.
Such cord will generally be a structure of steel wires, twisted appropriately, the wires having a diameter ranging from 0.03 to 0.80 mm, in general in the range from 0.14 to 0.40 mm, and the steel being in general carbon steel (preferably 0.65 to 0.95% carbon) in its ferritic state, having a tensile strength of at least 2000 N/mm.sup.2 and an elongation at rupture of at least 1%, and preferably about 2%. The cord will generally further comprise, in order to obtain the necessary rubber adherability for reinforcement purposes, a rubber-adherable coating, such as copper, zinc, brass or ternary brass alloy, or a combination thereof, the coating having a thickness ranging from 0.05 to 0.40 micron, preferably from 0.12 to 0.22 micron. The coating can also be present in the form of a thin film of chemical primer material for ensuring good rubber penetration and adhesion.
The wires are twisted into a bundle according to a given structure, e.g. twisted strands or superposed layers, and this bundle may or may not be provided with a wrapping filament, helicoidally wound around the bundle. In defining below any twisting structure and number of filaments, this wrapping filament is not taken into consideration, and may or may not be present in addition.
For tire belt and carcass in particular, the requirements for a suitable cord structure are specifically: high tensile strength (which a.o. requires a structure with a minimum of cabling loss), good compactness (in order to obtain thin reinforcement plies, necessary specifically in the belt area of the tire), high fatigue resistance (by inter alia less fretting in the contact points between wires), and simple manufacturing method (for reduced costs). For this use, the cords generally have a steel cross-sectional area ranging from 0.5 to 3.5 mm.sup.2 for heavy truck tires, and from 0.15 to 0.5 mm.sup.2 for light truck tires.
For meeting these requirements, single-bundle n.times.1 structures have been proposed, e.g. 12.times.1-structure, in which all the wires are twisted in the same direction and with the same pitch. In these structures, the wires come to stack together in a compact configuration, contacting each other along a line instead of in cross-points, so that fretting is very low. The cord is also made in a simple way in a single twisting operation, and further shows a good resistance to cutting as reflected in an impact test. Such 12.times.1-cord can also be considered as having a core of three wires, surrounded by a layer of nine wires.
This cord however shows two major drawbacks. In the first place, it shows the phenomenon of "wire migration". The cords are generally used in practice in e.g. tire plies in the form of cut lengths of 35-55 cm, and in running tests of a tire, one or more wires have been found to shift lengthwise with respect to their neighbours, and emerge at one end of the cord, at one side of the ply over a certain length, puncturing through the rubber and damaging the tire. Secondly, it has been observed that the advantages of this cord are obtained at the expense of its reinforcing ability in rubber. The rupture strength of the bare cord, as obtained in an Instron tensile test, is normal. But when embedded in rubber, and measured between Zwick clamps, which take the cord by the rubber, and where the cord has to take up the tensile force from the rubber and redistribute this over the wires, the rupture strength is lower. This latter test corresponds more with the actual loading in the tire, and it shows that this cord is not as good for transmission of the tensile forces from the circumference wires to the core wires.